$\dfrac{1}{10} - \dfrac{3}{8} = {?}$
Answer: $ = {\dfrac{1 \times 4}{10 \times 4}} - {\dfrac{3 \times 5}{8 \times 5}} $ $ = {\dfrac{4}{40}} - {\dfrac{15}{40}} $ $ = \dfrac{{4} - {15}}{40} $ $ = -\dfrac{11}{40}$